This invention relates in general to electro-fluid interface devices and more particularly to electro-fluid servo valves that convert an electrical signal into an output pressure or fluid flow proportional to that signal.
In many applications, such as the control of hydraulic actuators by electronic control circuits, it is necessary to convert an electrical signal to a proportional fluid flow or pressure output. A well-known device for effecting this conversion is a proportional servo valve that employs a very high bandwidth torque motor to convert the electrical signal to the mechanical motion of the torque motor armature. The mechanical motion of the armature can drive a standard spool valve directly, or more typically, it operates against a pair of nozzles to create a pressure differential in the nozzles.
Frequently, the forces on the spool valve are sufficiently high that it is necessary to operate the valve in two stages. In the first stage the torque motor armature operating against nozzles produces a difference in pressure upstream of the nozzles. This pressure difference is then used to position a second stage spool valve. U.S. Pat. No. 2,625,136 to Moog discloses a representative valve of this type. A major disadvantage of these valves is that the torque motor parts must be machined to very close material and dimensional tolerances which significantly increases the cost of manufacture. Other problems include a susceptibility to contamination build up and difficulty in following small signal changes, especially near the zero output point, due to friction and hysteresis effects.
U.S. Pat. No. 3,552,437 to Blosser discloses an interface device which controls a fluid flow by electromagnetically positioning a ball or valve member to open or close a selected inlet port. Although this device avoids the problems of a Moog type of valve, it has a fundamental shortcoming in that it is strictly an "on-off" device and therefore cannot produce a fluid flow or pressure change proportional to an electrical input.
U.S. Pat. No. 3,431,934 to Riordan describes a proportional flow device that employs an oscillating valve element to control the fluid flow. A pair of plenum chambers develop a driving pressure for the oscillation. The valve element alternately seats on an outlet from one of the plenum chambers until the fluid pressure in the chamber builds up to a level sufficient to overcome the seating forces. Coils convert an electrical signal into an electromagnetic force on the valve element to influence the average position of the oscillating valve member and thereby establish corresponding changes in the output flow or pressure.
One difficulty with the Riordan device is that the dwell time when the valve element is seated significantly limits the oscillation frequency. Since this frequency must exceed the band pass of the fluid system in which the device is connected, the usefulness of the device is significantly limited, particularly for hydraulic control applications. Another problem is the wear or deterioration of the seat due to the constant slamming of the valve element against the seat.
Another disadvantage is that the Riordan device is designed for use principally with compressible fluids that will rapidly develop a pressure build up in the plenum chambers when the valve element is seated. This dependence on fluid compressibility makes the valve operation sensitive to changes in the temperature and condition of the fluid which significantly increases the difficulty of maintaining a steady state oscillation at a preselected frequency and amplitude. To operate with relatively incompressible fluids, Riordan provides bellows within the plenum chambers. These bellows, however, greatly reduce the frequency of oscillation of the element.
It is therefore a principal object of the invention to provide a proportional electro-fluid interface device that is rugged, highly reliable, and operates effectively in a wide range of fluid systems while having a low cost of manufacture.
A further object of this invention is to provide a proportional electro-fluid device that minimizes contamination build up, has excellent wear characteristics, a good sensitivity near zero output, and a fast response time.
Yet another object of the invention is to provide an electro-fluid interface device that can be accurately, reliably and conveniently tuned to and maintain a desired frequency and amplitude of oscillation.